qmp: introduce QMPCapability

There were no QMP capabilities defined.  Define the first capability,
"oob", to allow out-of-band messages.

After this patch, we will allow QMP clients to enable QMP capabilities
when sending the first "qmp_capabilities" command.  Originally we are
starting QMP session with no arguments like:

  { "execute": "qmp_capabilities" }

Now we can enable some QMP capabilities using (take OOB as example,
which is the only capability that we support):

  { "execute": "qmp_capabilities",
    "arguments": { "enable": [ "oob" ] } }

When the "arguments" key is not provided, no capability is enabled.

For capability "oob", the monitor needs to be run on a dedicated IO
thread, otherwise the command will fail.  For example, trying to enable
OOB on a MUXed typed QMP monitor will fail.

One thing to mention is that QMP capabilities are per-monitor, and also
when the connection is closed due to some reason, the capabilities will
be reset.

Also, touch up qmp-test.c to test the new bits.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-11-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: touch up commit message]
Signed-off-by: Eric Blake <eblake@redhat.com>

monitor.c

@@ -59,6 +59,7 @@
 #include "qapi/qmp/qjson.h"
 #include "qapi/qmp/json-streamer.h"
 #include "qapi/qmp/json-parser.h"
+#include "qapi/qmp/qlist.h"
 #include "qom/object_interfaces.h"
 #include "trace-root.h"
 #include "trace/control.h"
@@ -170,6 +171,7 @@ typedef struct {
      * mode.
      */
     QmpCommandList *commands;
+    bool qmp_caps[QMP_CAPABILITY__MAX];
 } MonitorQMP;
 
 /*
@@ -1042,8 +1044,42 @@ static void monitor_init_qmp_commands(void)
                          qmp_marshal_qmp_capabilities, QCO_NO_OPTIONS);
 }
 
-void qmp_qmp_capabilities(Error **errp)
+static void qmp_caps_check(Monitor *mon, QMPCapabilityList *list,
+                           Error **errp)
 {
+    for (; list; list = list->next) {
+        assert(list->value < QMP_CAPABILITY__MAX);
+        switch (list->value) {
+        case QMP_CAPABILITY_OOB:
+            if (!mon->use_io_thr) {
+                /*
+                 * Out-Of-Band only works with monitors that are
+                 * running on dedicated IOThread.
+                 */
+                error_setg(errp, "This monitor does not support "
+                           "Out-Of-Band (OOB)");
+                return;
+            }
+            break;
+        default:
+            break;
+        }
+    }
+}
+
+/* This function should only be called after capabilities are checked. */
+static void qmp_caps_apply(Monitor *mon, QMPCapabilityList *list)
+{
+    for (; list; list = list->next) {
+        mon->qmp.qmp_caps[list->value] = true;
+    }
+}
+
+void qmp_qmp_capabilities(bool has_enable, QMPCapabilityList *enable,
+                          Error **errp)
+{
+    Error *local_err = NULL;
+
     if (cur_mon->qmp.commands == &qmp_commands) {
         error_set(errp, ERROR_CLASS_COMMAND_NOT_FOUND,
                   "Capabilities negotiation is already complete, command "
@@ -1051,6 +1087,21 @@ void qmp_qmp_capabilities(Error **errp)
         return;
     }
 
+    /* Enable QMP capabilities provided by the client if applicable. */
+    if (has_enable) {
+        qmp_caps_check(cur_mon, enable, &local_err);
+        if (local_err) {
+            /*
+             * Failed check on any of the capabilities will fail the
+             * entire command (and thus not apply any of the other
+             * capabilities that were also requested).
+             */
+            error_propagate(errp, local_err);
+            return;
+        }
+        qmp_caps_apply(cur_mon, enable);
+    }
+
     cur_mon->qmp.commands = &qmp_commands;
 }
 
@@ -3896,14 +3947,29 @@ void monitor_resume(Monitor *mon)
         readline_show_prompt(mon->rs);
 }
 
-static QObject *get_qmp_greeting(void)
+static QObject *get_qmp_greeting(Monitor *mon)
 {
+    QList *cap_list = qlist_new();
     QObject *ver = NULL;
+    QMPCapability cap;
 
     qmp_marshal_query_version(NULL, &ver, NULL);
 
-    return qobject_from_jsonf("{'QMP': {'version': %p, 'capabilities': []}}",
-                              ver);
+    for (cap = 0; cap < QMP_CAPABILITY__MAX; cap++) {
+        if (!mon->use_io_thr && cap == QMP_CAPABILITY_OOB) {
+            /* Monitors that are not using IOThread won't support OOB */
+            continue;
+        }
+        qlist_append(cap_list, qstring_from_str(QMPCapability_str(cap)));
+    }
+
+    return qobject_from_jsonf("{'QMP': {'version': %p, 'capabilities': %p}}",
+                              ver, cap_list);
+}
+
+static void monitor_qmp_caps_reset(Monitor *mon)
+{
+    memset(mon->qmp.qmp_caps, 0, sizeof(mon->qmp.qmp_caps));
 }
 
 static void monitor_qmp_event(void *opaque, int event)
@@ -3914,7 +3980,8 @@ static void monitor_qmp_event(void *opaque, int event)
     switch (event) {
     case CHR_EVENT_OPENED:
         mon->qmp.commands = &qmp_cap_negotiation_commands;
-        data = get_qmp_greeting();
+        monitor_qmp_caps_reset(mon);
+        data = get_qmp_greeting(mon);
         monitor_json_emitter(mon, data);
         qobject_decref(data);
         mon_refcount++;